The present invention relates to an intumescent-ablator which is prepared in the form of multi-layers for fire retardant purposes.
Ablation is a process where, through a combination of thermal, chemical and mechanical degradation, a substance dissipates energy with an attendant loss of surface material. High temperature heat ablative structures have assumed considerable importance in the development, for example, of high speed aircraft and space vehicles, these objects undergoing severe high temperature conditions during re-entry into the earth's atmosphere. Intumescent compositions generally combine an intumescent agent with a suitable polymeric binder.
Such a composition, when heated to the temperature at which the agent intumesces, results in the composition expanding many times its original volume, forming a charred layer which provides a protective barrier from flame and heat.
Among conventional intumescent compositions are those containing phosphate derivatives that form an acid which reacts with the carbon in the organic binders. Others are nitroaromatic acids. Usually the compositions contain a polymeric binder having a lower melting temperature and which allows the intumescent agents to release gases at the decomposition temperature.
With the advent of the space age, a great deal of research was directed toward the formulation of ablative materials which would be useful in rocket propulsion and re-entry systems. Most of the research was directed towards intumescent-ablator coatings or the like which were designed for a short high temperature pulse of up to 250 seconds in length.
The present invention focuses on the use of a multi-layer intumescent-ablator for fire resistant purposes for a duration of time up to 30 minutes and more and which is useful for protecting various media objects and others, such as computer disks and diskettes, microfilms, photographic films, papers, etc. These materials, used for the purpose of fire assimilation, are exposed to heat according to a standard time-temperature curve known as ASTM E-119 and the cold face temperature is measured over time.
The development of the present invention takes into consideration various parameters involved in the intumescent-ablative reaction. The following parameters were considered, namely, conductivity of the material before being exposed to heat according to the standard time-temperature curve, the heat capacity and latent heat of the ablative material before being exposed to heat according to the standard time-temperature curve, the heat capacity and latent heat of the ablative material, the gas divergation factor, and change of boundary conditions versus time through the intumescence (FIG. 5). The assimilation is a progressing front which creates the combined reaction of intumescence and ablation where, through the process in the front area, the materials go through decomposition which, in some cases, results in structure transformation and phase change of water into vapor and gas released in the front area. Residues after the reaction takes place (or after the heat front has passed) are heat refractory materials. The important factors involved in this invention are as follows:
The time required for the front to reach a certain point will highly depend on the thickness as a power of 2 and, as a power of 1, the parameters of the material itself, i.e., latent heat, heat capacity and transformation energy.
Several techniques have been developed in the past for ablation of intumescence, each dealing with these phenomena separately. Few techniques have incorporated the intumescence with the endothermic reaction.
Hansen, et al., in U.S. Pat. No. 4,122,059, and Raevsky, et al. in U.S. Pat. No. 4,462,831, each stress the use of the ablative reaction only with different ablative materials where a single layer of hydrated iron oxide or hydrated aluminum sulfate was used with an organic or inorganic binder. Raevsky, et al., stressed the importance of a percentage of inorganic salt in the composition as a major parameter for protection time whereas an objective of this invention is to show that other parameters such as layer thickness and expansion play a more important role.
Langer, et al., in U.S. Pat. No. 4,273,879 stress the usage of a flexible heat expanding material comprising an intumescent component in granular form, and an organic binder material.
Swawko, et al., in U.S. Pat. No. 4,088,806, refer to the combination of an intumescent-ablator which is used for coating purposes mostly for short heat pulses.